Racemic compound and anti-ferroelectric liquid crystal composition containing the compound

ABSTRACT

A racemic compound of the formula (1) ##STR1## and an anti-ferroelectric liquid crystal composition consisting essentially of said racemic compound and one anti-ferroelectric liquid crystal compound of the formula (2) or a mixture of two or more compounds selected from anti-ferroelectric liquid crystal compounds of the formula (2), ##STR2## said composition of the present invention, exhibiting a small spontaneous polarization and a fast response so that a display device having a high display quality can be achieved.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a novel racemic compound, a novelanti-ferroelectric liquid crystal composition containing the compoundand a liquid crystal display device for which the composition is used.

(2) Description of the Related Art Including Information Disclosed Under37 CFR 1.97 and 1.98.

A liquid crystal display device has been so far applied to a small-sizeddisplay device of various types due to its low-voltage operability,low-power consumption and display capability with a thin screen.Further, the liquid crystal display device is recently practically beingapplied to the fields of information and office automation-relatedmachines and equipment and the field of television sets, andsimultaneously, it is being applied to various fields of other use.

Under the circumstances, energetic developments are under way forattaining a large-sized liquid crystal display device of higherperformance that has a higher display capacity and a higher displayquality than a conventional CRT display device.

Liquid crystals used in currently available liquid crystal displaydevices are nematic liquid crystals, and they are classified into simplematrix driven liquid crystal devices and active matrix driven liquidcrystal devices according to their driving methods.

Simple matrix driven liquid crystal display devices are producedadvantageously in view of a cost due to their simple structures.However, these devices have the problems that the contrast is low due toa cross-talk phenomenon, that driving in a large capacity is difficultand that the display of video frames at a high duty ratio is difficultdue to a low response speed. It is therefore necessary to break throughmany technical problems for attaining a large-sized liquid crystaldisplay device capable of displaying video frames at a high duty ratio.

On the other hand, active matrix driven liquid crystal devices use a TFT(thin film transistor) method as a main stream, but it is required toform thin film transistors for each pixel, and a large investment isrequired for high production technology and the construction of aproduction line. The active matrix driving method is therefore fardisadvantageous in view of a cost as compared with the simple matrixdriving method. However, the active matrix driven liquid crystal devicehas a high contrast since the cross-talk phenomenon which is a problemof the simple matrix driving method is few, and further, its responsespeed is high. Therefore, there can be attained a liquid crystal displaydevice which has a high image quality and is capable of displaying videoframes at a high duty ratio. For this reason, the TFT method among theactive matrix driving methods is gaining its position as a main stream.

At present, large-sized liquid crystal devices having a size of 10 to 20inches are being developed, while the problem of viewing angledependency, which is inherent to a device using the nematic liquidcrystal, is critically serious. Various technical studies have been madefor overcoming the viewing angle dependency, and as a result, displayingwith a viewing angle of about 140° has been possible without causing agray scale inversion. However, the contrast is still dependent greatlyupon a viewing angle, and at present, there cannot yet be obtained suchflat contrast characteristics with regard to the wide viewing angle asachieved in CRT.

Under the above circumstances, a liquid crystal display device using aferroelectric liquid crystal attracts attention as a fast responseliquid crystal display device. A surface stabilized ferroelectric liquidcrystal (SSFLC) device disclosed by Clark and Lagerwall attractsattention due to its high response speed and wide viewing angle whichhave not been available in the past. Its switching characteristics havebeen studied in detail, and a number of ferroelectric liquid crystalcompounds have been synthesized for optimizing various physical propertyconstants.

On the other hand, for accomplishing a practical device, there have beena number of technical barriers to be overcome such as difficulties inachieving a memory effect and controlling a layer structure due to thedifficulty in controlling an alignment, the destruction of an alignmentcaused by a mechanical shock, and the like, and these problems have beenovercome to produce a device as a product.

However, the ferroelectric liquid crystal display device still hasproblems that it cannot display colors since it cannot handle, inprinciple, a gray scale and that the display of video frames isdifficult because high speed response has not been attained yet.

Further, as another high speed response liquid crystal display device,the development of a device having a switching mechanism different fromthat of SSFLC is also under way. This is a liquid crystal display devicewhich utilizes switching among tri-stable states of a liquid crystalhaving an anti-ferroelectric phase (to be referred to as"anti-ferroelectric liquid crystal" hereinafter) (Japanese Journal ofApplied Physics, Vol. 27, pp. L729, (1988)).

The anti-ferroelectric liquid crystal has the following three stablestates.

That is, the above three stable states are two uniform states (Ur, Ul)observed in two ferroelectric crystal states and one third state.Chandani et al report that the above third state is ananti-ferroelectric phase (Japanese Journal of Applied Physics, vol. 28,pp. L1261 (1989) and Japanese Journal of Applied Physics, vol. 28, pp.L1265 (1989)).

The above switching among the tri-stable states is the firstcharacteristic of an anti-ferroelectric liquid crystal.

The second characteristic of the anti-ferroelectric liquid crystal isthat a sharp threshold is present with regard to an applied voltage.

Further, it has a memory effect when a proper bias voltage is set, whichis the third characteristic of the anti-ferroelectric liquid crystal.

Further, the fourth characteristic of the anti-ferroelectric liquidcrystal is that its layer structure can be easily switched when anelectric field is applied (Japanese Journal of Applied Physics, Vol. 28,pp. L119, (1989), and vol. 29, pp. L111 (1990)). Owing to thischaracteristic, a liquid crystal display device almost free of defectsand having self-restoring ability of the alignment can be produced.

By utilizing those characteristics described above, a liquid crystaldevice having a high response speed and an excellent contrast can beachieved.

Further, it has been demonstrated that the gray shade display, which isalmost impossible to achieve with a ferroelectric liquid crystal device,is possible to achieve with an anti-ferroelectric liquid crystal device.It has been consequently made possible to shift toward a full-colordisplay, and the importance of an anti-ferroelectric liquid crystal isfurther increasing (Preprints of No. 4 Ferroelectric Liquid CrystalInternational Symposium, page 77, (1993)).

BRIEF SUMMARY OF THE INVENTION

Under the circumstances, energetic developments are under way forachieving an anti-ferroelectric liquid crystal display device, but thedevelopments for achieving practical devices are presently encounteringthe following problems.

When an anti-ferroelectric liquid crystal is used as a display device,generally, the anti-ferroelectric liquid crystal is sandwiched betweentwo glass substrates coated with an insulation layer and an alignmentlayer.

The insulation layer is necessary for preventing a short circuit betweenthe substrates, and it is required to have a certain thickness for thecomplete prevention of a short circuit. On the other hand, the alignmentlayer is required for aligning liquid crystal molecules in onedirection, and it is also required to have a certain thickness forreducing alignment defects, which occur when the liquid crystalmolecules are aligned, to the least degree.

When a voltage is applied to the thus formed liquid crystal device, thephase transition from an anti-ferroelectric state to a ferroelectricstate occurs sharply with regard to the applied voltage when theinsulation layer and the alignment layer have a small thickness or whenthe insulation layer or the alignment layer is completely absent.However, when the insulation layer and the alignment layer have acertain thickness required for practical use, the phase transition fromthe anti-ferroelectric state to the ferroelectric state takes placemoderately with regard to the applied voltage.

In the driving of an anti-ferroelectric liquid crystal, a holdingvoltage lower than a writing voltage is continuously applied for apredetermined period of time after the writing voltage for producing amemory effect is applied. When the phase transition from ananti-ferroelectric state to a ferroelectric state takes place moderatelywith regard to an applied voltage as described above, that is, when aliquid crystal display device has a low steepness of threshold, theholding voltage that can be selected is limited to a very narrow range,and in an extreme case, the holding voltage cannot be set, and no memoryeffect is secured. This means that an anti-ferroelectric liquid crystaldisplay device is no longer useful as such, which is a serious problem.

Further, the lower the steepness of threshold in the device, thenarrower the range of the holding voltage that can be selected, and aso-called driving margin decreases accordingly. A practical device istherefore required to have a high steepness of threshold, and liquidcrystal materials which can give such a steepness of threshold aregradually being demanded.

Practically, an anti-ferroelectric liquid crystal is preferably amaterial that can provide a high steepness of threshold as describedabove when used in a liquid crystal device.

It has been experimentally found that the steepness of threshold of theliquid crystal device is closely related to the thickness of both theinsulation layer and the alignment layer.

Studies have been made to determine what factors can explain the aboverelationship. In the following studies, both an insulation layer and analignment layer will be together referred to as an "alignment layer".

For easier understanding of the studies, the studies will be explainedwith reference to FIGS. 1 to 3 below.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is an equivalent circuit of an anti-ferroelectric liquid crystaldevice.

FIG. 2 shows a simulation result on the steepness of threshold whenalignment layer is present.

FIG. 3 shows a simulation result on the steepness of threshold when analignment layer is present.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an equivalent circuit which comprises an electric currentsource to generate a polarization inversion current according to anapplied voltage, an alignment layer that is an electrostatic capacitor Cto connect with an anti-ferroelectric liquid crystal in series, and adriving circuit that is an ideal voltage source.

In FIG. 1, the drive voltage applied to a device is taken as Vex, avoltage generated between the upper and lower surfaces of an alignmentlayer by the charge of a polarization inversion current is taken as Vc,an effective voltage to be actually applied to the liquid crystal istaken as Veff, a spontaneous polarization of the liquid crystal is takenas P, an electrode area of the liquid crystal device is taken as S, athickness of the alignment layer is taken as d', and a dielectricconstant of the alignment layer is taken as ε'. Vc is calculated as inthe following equation (1).

    Vc=PS/C=PSd'/(Sε')=P(d'/ε')                (1)

On the basis of the above equation, Veff is expressed as in thefollowing equation (2).

    Veff=Vex-Vc=Vex-P(d'/ε')                           (2)

As is clear from the equation (2), the voltage actually applied to theliquid crystal is lower than the externally applied voltage by a productof the polarization P of the liquid crystal, the thickness d' of thealignment layer and a reciprocal number 1/ε' of the dielectric constantof the alignment layer.

Then, when a thickness of the liquid crystal layer filled in a liquidcrystal cell is taken as d, an electric field Eeff actually applied tothe liquid crystal is expressed by the following equation (3).

    Eeff=Veff/d                                                (3)

On the other hand, an apparent electric field strength Eex is expressedby the following equation (4).

    Eex=Vex/d=(Veff+Vc)/d=Veff/d+P(d'/ε')/d=Eeff+αP(4)

wherein

    α=d'/(ε'd)                                   (5)

When no alignment layer is present, the second term in the equation (4)is 0, and hence Eex=Eeff.

While an anti-ferroelectric liquid crystal shows a hysteresis of itsoptical response with regard to an applied voltage, four thresholds arethinkable with regard to the hysteresis.

Each threshold is Eeff (=Eex), and in this case, these thresholds do notincline to an electric field. FIG. 2 shows this appearance.

When an alignment layer is present, the equation (4) is modified toobtain the following equation (6).

    Eeff=Eex-αP                                          (6)

That is, an effective electric field exerting on the liquid crystal islower than the applied electric field Eex by α·P. As a result, thehysteresis is strained to a great extent due to the contribution of theα·P as shown in FIG. 3.

The above studies show that the strain of hysteresis is greatly causedby the interaction of the spontaneous polarization and the alignmentlayer. For obtaining a liquid crystal device having the reduced strainof hysteresis, therefore, it is effective to decrease the aboveinteraction so as to make it as small as possible.

The measures that can be specifically taken for achieving the abovepurpose include a means to use an alignment layer having a highdielectric constant, a means to decrease the thickness of the alignmentlayer or to decrease the spontaneous polarization of the liquid crystal,as is clear from the above equations (5) and (6). Of the above measures,it is rather difficult to acquire an alignment layer material having ahigh dielectric constant and hence, the measures that can be practicallytaken is a means to decrease the thickness of the alignment layer or todecrease the spontaneous polarization of the liquid crystal material.

Generally, an anti-ferroelectric liquid crystal compound has aconsiderably large spontaneous polarization, and a liquid crystalmaterial having relatively excellent physical properties has aspontaneous polarization of 200 nC/cm² or more. Therefore, unless thethickness of the alignment layer is much decreased, the strain of thehysteresis is considerably large. However, when the thickness of thealignment layer is decreased, there occurs a problem that the alignmentstate of the liquid crystal molecules is too defective to procure acontrast. The measure for correcting the strain of the hysteresis bydecreasing the thickness of the alignment layer is thereforeconsiderably limited.

On the other hand, in order to decrease the spontaneous polarization ofa liquid crystal material, it is inevitable to take a means to mix aproper compound having no spontaneous polarization with the liquidcrystal material, that is, to dilute the liquid crystal material todecrease its concentration. Since, however, the response speed of aliquid crystal is determined by a product of an applied voltage and aspontaneous polarization, there occurs another new problem that theresponse speed decreases when the spontaneous polarization is decreasedsimply by mixing with a proper compound.

Under the circumstances, for obtaining a device having a decreasedstrain of hysteresis, attempts have been so far made to develop ananti-ferroelectric liquid crystal having a low spontaneous polarization,a low threshold voltage and a low viscosity, but it is a currentsituation that no satisfactory achievements have been obtained.

The present invention has been made from the above points of view, andhas been completed by finding the following. By selecting and adding aracemic compound having a novel chemical structure to ananti-ferroelectric liquid crystal, the spontaneous polarization can bedecreased without decreasing the response speed, and when thecomposition is used for forming a liquid crystal device, a liquidcrystal device having a decreased strain of hysteresis can be obtained.

That is, according to the present invention, there is provided a racemiccompound of the following general formula (1), ##STR3## wherein m is aninteger of 8 to 10, n is an integer of 3 to 8, and X is a hydrogen atomor a fluorine atom.

Further, according to the present invention, there is provided ananti-ferroelectric liquid crystal composition consisting essentially ofone or more of the racemic compound of the following general formula (1)##STR4## wherein m is an integer of 8 to 10, n is an integer of 3 to 8,and X is a hydrogen atom or a fluorine atom,

and an anti-ferroelectric liquid crystal compound of the followingformula (2), ##STR5## wherein R is a linear alkyl group having 6 to 12carbon atoms, Z is a hydrogen atom or a fluorine atom, A is --CH₃ or--CF₃, r is 0 or 1 and C* is an asymmetric carbon atom, provided thatwhen A is --CH₃, r is 0 and p is an integer of 4 to 10, that when A is--CF₃ and r is 0, p is an integer of 6 to 8 and that when A is --CF₃ andr is 1, s is an integer of 5 to 8 and p is an integer of 2 or 4.

The present invention will be more specifically explained hereinafter.

In the above general formula (1) for the racemic compound of the presentinvention, m is an integer of 8 to 10, preferably 9, and n is an integerof 3 to 8, preferably 6. Further, X is a hydrogen atom or a fluorineatom, preferably a fluorine atom.

The racemic compound of the above general formula (1) can be produced bythe following method, for example. ##STR6##

The production method of the racemic compound of the above generalformula will be briefly explained below.

(1) shows the etherfication by the reaction of alkyl bromide and4-hydroxybenzoic acid.

(2) shows the conversion of 4-acetoxy-2-fluorobenzoic acid to chloride.

(3) shows the esterification by the reaction of acid chloride and2-alkyl alcohol.

(4) shows deacetylation.

(5) shows the conversion of p-alkoxybenzoic acid to chloride.

(6) shows an esterification with acid chloride (formation of an endproduct).

The anti-ferroelectric liquid crystal composition consists essentiallyof the racemic compound of the general formula (1) and theanti-ferroelectric liquid crystal compound of the general formula (2).

In the general formula (2), R is a linear alkyl group having 6 to 12carbon atoms, preferably 8 to 10 carbon atoms, Z is a hydrogen atom or afluorine atom, A is a --CH₃ or --CF₃, and r is 0 or 1. Further, thedefinitions of p and s vary depending upon the kind of A and the valueof r. That is, when A is --CH₃, r is 0 and p is an integer of 4 to 10.When A is --CF₃ and r is 0, p is an integer of 6 to 8. When A is --CF₃and r is 1, s is an integer of 5 to 8 and p is an integer of 2 or 4.

As a compound of the general formula (2), preferred are a compound ofthe general formula (2) in which A is --CF₃ and r is 1 and a compound ofthe general formula (2) in which A is --CH₃, r is 0 and p is an integer.of 4 to 6.

The anti-ferroelectric liquid crystal compound of the general formula(2) can be easily produced, for example, by the following method. Acompound of the general formula (2) in which A=--CF₃, p=2, r=1 and s=5is produced by the following method.

(a) AcO--Ph(Z)--COOH+SOCl₂ →AcO--Ph(Z)--COCl

(b) (a)+HOC*H(CF₃)(CH₂)₅ OC₂ H₅ →AcO--Ph(Z)--COOC*H(CF₃)(CH₂)₅ OC₂ H₅

(c) (b)+Ph--CH₂ NH₂ →HO--Ph(Z )--COOC*H (CF₃ )(CH₂)₅ OC₂ H₅

(d) RO--Ph--Ph--COOH+SOCl₂ →RO--Ph--Ph--COCl

(e) (b)+(d)→anti-ferroelectric liquid crystal compound

In the above formulae, AcO-- is an acetyl group, --Ph(Z)-- is a1,4-phenylene group in which fluorine may be substituted, Ph-- is aphenyl group, --Ph-- is a 1,4-phenylene group and C* is an asymmetriccarbon atom.

The above production method will be briefly explained below.

(a) shows the chlorination of fluorine-substituted or non-substitutedp-acetoxybenzoic acid with thionyl chloride.

(b) shows a reaction between a chlorination product obtained in (a) andan alcohol to form an ester.

(c) shows the deacetylation of the ester obtained in (b).

(d) shows the chlorination of 4'-alkyloxybiphenyl-4-carboxylic acid.

(e) shows the formation of a liquid crystal by a reaction between aphenol obtained in (c) and a chlorination product obtained in (d).

The anti-ferroelectric liquid crystal composition of the presentinvention consists essentially of the racemic compound of the abovegeneral formula (1) and the anti-ferroelectric liquid crystal compoundof the general formula (2). Specifically, it is advantageous that thetotal amount of the compounds of the formulae (1) and (2) based on thetotal composition is at least 70 mol %, preferably at least 80 mol %.

The mixing ratio ((1):(2))of the compound of the above general formula(1) to the compound of the above general formula (2) is preferably inthe range of 1:99 to 40:60, particularly preferably 5:95 to 35:65, interms of a molar ratio.

Further, a compound or a mixture of at least two compounds of the abovegeneral formula (2) may be used. The use of a mixture of at least twocompounds of the formula (2) can give a liquid crystal display devicewhich is excellent in alignment characteristic and steepness ofthreshold and exhibits a high contrast.

In the anti-ferroelectric liquid crystal composition of the presentinvention, preferably, the upper limit of temperature range of theanti-ferroelectric phase is at least 40° C., the lower limit thereof is0° C. or lower, and at least a smectic A phase is present outside atemperature range higher than the temperature range in which theanti-ferroelectric phase is present. The anti-ferroelectric liquidcrystal composition of the present invention is preferably used in ananti-ferroelectric liquid crystal display device formed by interposingthe composition between a pair of electrode substrates.

The present invention can provide a novel racemic compound and a novelanti-ferroelectric liquid crystal composition containing the aboveracemic compound. Further, the novel anti-ferroelectric liquid crystalcomposition of the present invention can provide an anti-ferroelectricliquid crystal display device which is excellent in steepness ofthreshold, has an anti-ferroelectric phase over a broad temperaturerange and exhibits a high speed response, and which therefore has a highdisplay quality.

EXAMPLES

The present invention will be explained more specifically with referenceto Examples and Comparative Examples hereinafter, while the presentinvention shall not be limited thereto.

Example 1

Preparation of3-fluoro-4-(1-methylheptyloxycarbonyl)phenyl=4-n-nonyloxybenzoate(Formula (1): m=9, n=6, X=F (E1))

(1) Preparation of p-nonyloxybenzoic acid

12.7 Grams (0.0917 mol) of p-hydroxybenzoic acid, 28.5 g of n-nonylbromide and 10.2 g of potassium hydroxide were added to a mixturecomposed of 1,500 ml (milliliter) of ethanol and 200 ml of water, andthe mixture was allowed to react under reflux for 10 hours. Further, 500ml of water was added thereto, and the mixture was stirred for 3 hours.After the completion of the reaction, a concentrated hydrochloric acidwas added to acidify the reaction mixture. Thereafter, the solvent wasdistilled off, and the remaining product was cooled to room temperatureand then filtered to give a colorless solid. The solid was fully washedwith water and re-crystallized from chloroform to give an intendedproduct (yield 75%).

(2) Preparation of4-acetoxy-2-fluoro-1-(1-methylheptyloxycarbonyl)benzene

To 10.8 g (0.06 mol) of 4-acetoxy-2-fluorobenzoic acid was added 60 mlof thionyl chloride, and the mixture was allowed to react under refluxfor 7 hours. Then, excess thionyl chloride was distilled off, and 10 mlof pyridine and 5.3 g (0.0402 mol) of 2-octanol were added dropwise. Theresultant mixture was stirred at room temperature for a whole day, andthen diluted with 200 ml of ether. An organic layer was washed withdilute hydrochloric acid, with a 1N sodium hydroxide aqueous solutionand with water in this order, and dried over magnesium sulfate.

The solvent was distilled off, and the resultant crude intended productwas purified with silica gel column chromatograph using hexane/ethylacetate as solvents, to give an intended product (yield 90%).

(3) Preparation of2-fluoro-4-hydroxy-1-(1-methylheptyloxycarbonyl)benzene

9.7 Grams (0.0361 mol) of the compound obtained in the above (2) wasdissolved in 250 ml of ethanol, and 7.7 g (0.0772 mol) of benzylaminewas added dropwise. Further, the mixture was stirred at room temperaturefor a whole day and diluted with 300 ml of ether, and the dilutedmixture was washed with dilute hydrochloric acid and with water in thisorder, and dried over magnesium sulfate. The solvent was distilled off,and an intended product was isolated and purified by silica gel columnchromatography (yield 98%).

(4) Preparation of3-fluoro-4-(1-methylheptyloxycarbonylphenyl)=4-n-nonyloxybenzoate

To 3.1 ml of the compound obtained in the above (1) was added 15 ml ofthionyl chloride, and the mixture was refluxed under heat for 5 hours.Excess thionyl chloride was distilled off, then, 2 ml of pyridine and2.12 mmol of the compound obtained in the above (3) were added, and themixture was allowed to react at room temperature for 10 hours.

After the completion of the reaction, the reaction mixture was dilutedwith 300 ml of ether, the diluted mixture was washed with dilutehydrochloric acid, with a 1N sodium carbonate aqueous solution and withwater in this order, and an organic layer was dried over magnesiumsulfate.

Then, the solvent was distilled off, and an intended product wasisolated by silica gel column chromatography (yield 81%).

Table 1 shows NMR data of the compound obtained in Example 1, and theformula of the compound is shown as the formula, (E1).

                  TABLE 1                                                         ______________________________________                                               Chemical shift                                                                  1H      2H    3H    4H  5H     6H  7H                                ______________________________________                                        Example 1 (E1)                                                                         4.1     7.0   7.1   7.1 7.1    8.0 5.2                               ______________________________________                                         ##STR7##

Comparative Example 1

Anti-ferroelectric liquid crystal compounds (2A and 2B) of the followingformulae were mixed in a mixing ratio of 70/30 (molar ratio) to obtainan anti-ferroelectric liquid crystal composition.

Table 2 shows the phase sequence of the obtained composition. Further,the liquid crystal composition was measured for a spontaneouspolarization at 60° C. and a response time in the transition from ananti-ferroelectric state to a ferroelectric state. Table 2 shows theresults.

2A: C₉ H₁₉ O--Ph--Ph--COO--Ph(3F)--COO--C*H(CF₃)(CH₂)₅ OC₂ H₅ (Formula(2): R=C₉ H₁₉, Z=F, A=CF₃, r=1, s=5 and p=2)

2B: C₈ H₁₇ O--Ph--Ph--COO--Ph(3F)--COO--C*H(CH₃)C₅ H₁₁ (Formula (2):R=C₈ H₁₇, Z=F, A=CH₃, r=0 p=5)

The above composition was measured or evaluated for an optical responsehysteresis, a response time and a spontaneous polarization as follows.

A liquid crystal cell (cell thickness 2 μm) having ITO electrodes and arubbed polyimide thin film (30 nm) was charged with a liquid crystalcomposition in an isotropic state. Then, the cell was gradually cooledat a rate of 1.0° C./minute to align the liquid crystal. The cell wasinterposed between the crossed polarizers such that the layer directionof the liquid crystal was in parallel with an analyzer or a polarizer.

The response time in a transition from an anti-ferroelectric state to aferroelectric state was defined to be an amount of time required for achange in transmittance from 10% to 90% when a maximum transmittancerepresented 100%, and a minimum transmittance represented 0%, under theapplication of 25 V having a frequency of 10 Hz at 60° C.

The spontaneous polarization was determined by applying a 25 Vtriangular wave at 60° C. and measuring a polarization inversioncurrent.

Example 2

The anti-ferroelectric liquid crystal composition containing theanti-ferroelectric liquid crystal compounds (2A and 2B), used inComparative Example 1, was mixed with the racemic compound (E1) obtainedin Example 1 in a 2A/2B/E1 mixing ratio of 56/24/20 (molar ratio), toobtain an anti-ferroelectric liquid crystal composition. The so-obtainedcomposition was measured or evaluated for a phase sequence, aspontaneous polarization and a response time in the same manner as inComparative Example 1. Table 2 shows the results.

Though the spontaneous polarization was decreased, a higher responsecapability was exhibited.

                  TABLE 2                                                         ______________________________________                                                              Spontaneous                                                                             Response                                         polarization time                                                            Phase sequence (nC/cm.sup.2) (μsecond)                                   ______________________________________                                        Ex. 2 Cr(<-20)SCA*(76)SA(92)I                                                                           116       38                                          C Ex. 1 Cr(<-10)SCA*(95)SC*(97)SA(105)I 172 53                              ______________________________________                                         Ex. = Example, C Ex. = Comparative Example                               

In the phase sequence, parenthesized values show transition temperatures(°C.), Cr is a crystal phase, SCA* is an anti-ferroelectric phase, SC*is a ferroelectric phase, SA is a smectic A phase, and I is an isotropicphase.

What is claimed is:
 1. An anti-ferroelectric liquid crystal compositionconsisting essentially of one or more of a racemic compound of thegeneral formula (1), ##STR8## wherein m is an integer of 8 to 10, n isan integer of 3 to 8, and X is a hydrogen atom or a fluorine atom,and ananti-ferroelectric liquid crystal compound of the following formula (2),##STR9## wherein R is a linear alkyl group having 6 to 12 carbon atoms,Z is a hydrogen atom or a fluorine atom, A is --CH₃ or --CF₃, r is 0 or1 and C* is an asymmetric carbon atom, provided that when A is --CH₃, ris 0 and p is an integer of 4 to 10, that when A is --CF₃ and r is 0, pis an integer of 6 to 8 and that when A is --CF₃ and r is 1, s is aninteger of 5 to 8 and p is an integer of 2 or
 4. 2. The composition ofclaim 1, wherein m in the general formula (1) is
 9. 3. The compositionof claim 1, wherein n in the general formula (1) is
 6. 4. Thecomposition of claim 1, wherein X in the general formula (1) is afluorine atom.
 5. The composition of claim 1, wherein in the generalformula (2), A is --CH₃, r is 0 and p is an integer of 4 to
 6. 6. Thecomposition of claim 1, wherein the composition contains the racemiccompound of the general formula (1) and the anti-ferroelectric liquidcrystal compound of the general formula (2) in a molar ratio ((1):(2))of 1:99 to 40:60.
 7. The composition of claim 1, wherein an upper limitof temperature range of an anti-ferroelectric phase of the compositionis at least 40° C., a lower limit thereof is 0° C. or lower, and atleast a smectic A phase is present outside a temperature range higherthan the temperature range in which the anti-ferroelectric phase ispresent.
 8. Anti-ferroelectric liquid crystal display device comprisingthe anti-ferroelectric liquid crystal composition recited in claim 1,which is interposed between a pair of electrode substrates.
 9. Ananti-ferroelectric liquid crystal composition consisting essentially ofone or more of a racemic compound of the general formula (1), ##STR10##wherein m is an integer of 8 to 10,n is an integer of 3 to 8, and X is ahydrogen atom or a fluorine atom and an anti-ferroelectric liquidcrystal compound of the following formula (2) ##STR11## wherein R is alinear alkyl group having 6 to 12 carbon atoms, Z is a hydrogen atom ora fluorine atom, A is --CF₃, r is 1, C* is an asymmetric carbon atom sis an integer of 5 to 8, and p is an integer of 2 or 4.